Machining centers or systems incorporating an electroerosion machine, e.g. EDM (electrical discharge machining) machine, have already been known and increasingly adopted in the industry to improve the efficiency of machining operations. In these systems, a tool storage station is employed to store a multiplicity of formed tool electrodes to be interchangeably emloyed in the machining station. A plurality of formed electrodes of a similar shape and identical or slightly different size have been necessary to compensate for the erosive wear of each individual tool electrode and further to machine a workpiece with progressively increasing surface finishes. A plurality of such a set of formed tool electrodes has also been found necessary to complementarily shape various portions of a single or several three-dimensional contours to be imparted to a single workpiece. These tool electrodes must each individually be formed with precision, entailing time-consuming, laborious and costly jobs which render the conventional electroerosive machining centers still inefficient. Each tool electrode precision-formed to achieve a particular machining process loses its geometry due to wear in the process and becomes no longer useful for that process. Thus, in the use of any conventional electroerosive machining center a significantly high proportion of the cost for a given set of machining operations must have been expended for obtaining tool electrodes.